1. Field of the Invention
The invention relates to a process for iodinating hydroxyaromatic and aminoaromatic compounds economically and under mild conditions using a metal iodide and a metal hypochlorite.
2. Discussion of Background
Iodinated hydroxyaromatic compounds are useful intermediates in the production of hydroxybenzoic acids, which have commercial utility as monomers in the production of liquid crystal polyesters. See for example, U.S. Pat. No. 3,549,593.
Hydroxybenzoic acids may be produced by iodinating a phenolic compound and subsequently carbonylating the iodophenol to produce a hydroxybenzoic acid.
One method of iodinating hydroxyaromatic compounds utilizes an oxyiodination reaction in the presence of zeolite catalysts. See U.S. application Ser. No. 029,897 filed Mar. 25, 1987. The aromatic starting material is passed over the catalyst at relatively high temperatures of between about 100.degree.-500.degree. C. Additionally, starting materials containing alkyl groups cannot be used in the process since the alkyl groups are usually oxidized during the process.
Iodination of hydroxyaromatic compounds using sodium iodide and tertiary-butyl hypochlorite is known. See J. Org. Chem., 50 5384-5387 (1985). This method is undersirable, however, since t-butyl hypochlorite is expensive and unstable and therefore inconvenient to use. Additionally, the method utilizes expensive and toxic solvents such as acetonitrile, and the reaction mixture must be buffered which significantly increases the cost when using the method in an industrial process. Additionally, this method is somewhat non-selective since frequently both mono- and di-substituted products are produced.
Finally, the use of tertiary-butyl hypochlorite appears to be limited to iodinations of phenolic compounds and has not been generalized to other activated aromatic compounds such as aromatic amines, i.e. anilines, hydroxypyridines, or hydroxyquinolines.
Selectivity is important in the iodination of hydroxy and aminoaromatic compounds since this process may be utilized to introduce a radioactive iodine such as for example 131.sub.I or 125.sub.I into a compound as a radiolabel. Since these iodine isotopes hava a relatively short half life (8 days for 131.sub.I and 60 days for 125.sub.I) and are expensive, a method of introducing the radiolabel must be both efficient and economical. That is, the reaction must be fast and be very selective.
High selectivity is also important since iodinated aromatics are useful intermediates that can be carbonylated or coupled to various polymer intermediates. The carbonylation of p-iodophenol to p-hydroxybenzoic acid is one example where high selectivity to the para isomen is desired.
Known methods of iodinating hydroxyaromatic compounds are relatively non-selective and are therefore inappropriate for such applications.
Accordingly, there exists a need for a selective method of iodination hydroxyaromatic and aminoaromatic compounds.
A further need exists for an iodination method which utilizes inexpensive and nontoxic reagents.
A further need exists for an iodination method which has a short reaction time and which is very selective and therefore is useful in radiolabelling applications.